Glass composition and cooking appliance

ABSTRACT

Provided is a glass composition comprising a glass frit containing P 2 O 5 , SiO 2 , B 2 O 3 , Al 2 O 3 , ZrO 2  and a group I-based oxide, wherein the P 2 O 5  is contained in an amount of 10 to 30% by weight based on a total weight of the glass frit, wherein the SiO 2  is contained in an amount of 20 to 40% by weight based on the total weight of the glass frit, wherein the B 2 O 3  is contained in an amount of 5 wt % to 18 wt % based on the total weight of the glass frit, wherein the Al 2 O 3  is contained in an amount of 15 to 30% by weight based on the total weight of the glass frit, wherein the ZrO 2  is contained in an amount of 1 wt % to 8 wt % based on the total weight of the glass frit, wherein the Group I-based oxide is contained in an amount of 15 to 30% by weight based on the total weight of the glass frit.

TECHNICAL FIELD

Embodiments relates to a glass composition, and a cooking appliance including the composition.

BACKGROUND ART

Enamel refers to a glass glaze coated on a surface of a metal plate. Enamel is typically used for cooking appliances. Such as microwave ovens and ovens. Enamel may be classified as an oxidation-resistant enamel which can prevent oxidation, or a heat-resistant enamel which can withstand high temperature, depending on the kind or use of glaze. Depending on the material added into the enamel, enamel may be classified into aluminum enamel, zirconium enamel, titanium enamel, and soda glass enamel.

In general, a cooking appliance refers to a device that heats and cooks food using a heating source. The food waste generated in the cooking process contacts the inner wall of the cavity portion of the cooking appliance. Therefore, when the food is cooked in the cooking appliance, it is necessary to clean the inside of the cavity portion. In addition, because cooking generally involves high temperature, the inner wall of the cavity portion may be exposed to the organic substance and the alkali ingredient. Therefore, when enamel is used therein, such enamel requires heat resistance, chemical resistance, abrasion resistance, and contamination resistance. Therefore, there is a need for a composition for enamel for improving the heat resistance, chemical resistance, abrasion resistance, and contamination resistance of enamel.

Particularly, in order to easily clean the enamel used in the oven, a pyrolysis method of burning contaminants at high temperature, a method of using a strong alkaline detergent, or a water soaking may be used. Accordingly, because the enamel is exposed to a high-temperature or high-alkali detergent, the enamel requires high heat resistance and chemical resistance.

Embodiments provide a glass composition with improved cleaning performance and improved reliability, and a cooking appliance using the same

In one aspect, there is provided a glass composition comprising a glass frit containing P₂O₅, SiO₂, B₂O₃, Al₂O₃, ZrO₂ and a group I-based oxide, wherein the P₂O₅ is contained in an amount of 10 to 30% by weight based on a total weight of the glass frit, wherein the SiO₂ is contained in an amount of 20 to 40% by weight based on the total weight of the glass frit, wherein the B₂O₃ is contained in an amount of 5 wt % to 18 wt % based on the total weight of the glass frit, wherein the Al₂O₃ is contained in an amount of 15 to 30% by weight based on the total weight of the glass frit, wherein the ZrO₂ is contained in an amount of 1 wt % to 8 wt % based on the total weight of the glass frit, wherein the Group I-based oxide is contained in an amount of 15 to 30% by weight based on the total weight of the glass frit.

According to the present disclosure, the glass composition may have improved cleanability.

Accordingly, the cleaning of the cooking appliance may be improved. In detail, the cooking appliance can be easily cleaned only with water-soaking.

That is, although the inner face of the door and the inner face of the cavity portion are contaminated by food and organic substances generated during the cooking process while the cooking space defined in the cavity portion of the cooking appliance is closed, the inner face of the door and the inner face of the cavity portion may be easily cleaned only with the water-soaking.

Further, since the functional layer coated using the glass composition according to the embodiment is excellent in cleaning property, the interior of the cooking appliance can be easily cleaned even with a small energy.

Further, contaminants including oil, such as chicken oil, and sugars can be effectively removed with less energy and less time, from the functional layer coated using the glass composition according to the embodiment.

Further, since the function layer can be directly coated on the cavity and the back surface of the door without a separate buffer layer, the process efficiency can be improved while minimizing the increase in the thickness of the cavity and the door due to the coating of the functional layer.

Further, since the glass composition according to the embodiment has a softening point and thermal expansion coefficient higher than a certain temperature, the function face may withstand cooking and washing at a high temperature for a long time.

Further, in the glass composition according to the embodiment, the weight percentage of Al₂O₃ may be appropriately controlled to reduce the caramelization reaction which occurs when the sugar component comes into contact with the functional layer made of the glass composition.

Therefore, it is possible to prevent the characteristics of the functional layer from being changed or deteriorated by the oxidation reaction such as caramelization reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a cooking appliance according to an embodiment.

FIG. 2 and FIG. 3 are enlarged cross-sectional views of a portion of the inner face of the cavity portion of FIG. 1.

FIG. 4 and FIG. 5 are enlarged views of a portion of the inner face of the door of FIG. 1.

FIG. 6 is a view for illustrating that contaminants are removed from the coating layer.

DETAILED DESCRIPTIONS

Hereinafter, a glass composition according to an embodiment and a cooking appliance including the same will be described with reference to the drawings.

Of the terms described below, adherence may be defined as the pre-firing coatability of the glass composition, and adhesion may be defined as post-firing coatability thereof.

FIG. 1 is a front view of a cooking appliance according to an embodiment.

Referring to FIG. 1, a cooking device or appliance 1 may include a cavity portion 11 having a cooking space 12 defined therein; a door 14 configured to selectively open and close the cooking space 12; and at least one heat source configured to supply heat for heating food to be cooked within the cooking space 12.

In detail, the cavity portion 11 may be formed in a hexahedral shape with the front face opened. The heat source includes the following components: a convection assembly 13 for discharging heated air into the cavity in the cavity portion 11, an upper heater 15 disposed on the upper inner face of the cavity portion 11, and a lower heater 16 disposed on a lower inner face of the cavity portion 11. Of course, the heat source does not necessarily have to include all of the convection assembly 13, the upper heater 15 and the lower heater 16. That is, the heat source may include at least one of the convection assembly 13, the upper heater 15, and the lower heater 16.

The upper heater 15 and/or the lower heater 16 may be provided inside or outside the cavity of the cavity portion 11.

Referring to FIG. 2 to FIG. 5, a functional layer may be disposed on the inner face of the cavity portion 11 and/or on the inner face of the door 14.

The functional layer may comprise a glass composition as described below. The functional layer may be coated on the inner face of the cavity portion 11 and/or the inner face of the door 14. That is, the functional layer may be a coating layer.

The functional layer can improve the heat resistance, chemical resistance and stain resistance of the inner face of the cavity portion 11 and the inner face of the door 14.

Referring to FIGS. 2 and 3, the functional layer may be disposed on the cavity portion.

The cavity portion 11 may include a metal layer 11 a, and a functional layer 11 b on the metal layer 11 a.

The metal layer 11 a may be the base of the cavity portion.

Referring to FIG. 2, the functional layer 11 b may be disposed in direct contact with the metal layer 11 a.

Alternatively, referring to FIG. 3, the functional layer 11 b may be in indirect contact with the metal layer 11 a. In detail, a buffer layer 11 c may be disposed between the metal layer 11 a and the functional layer 11 b. The buffer layer 11 c may comprise an adhesive layer. That is, by the buffer layer 11 c, the adhesion between the metal layer 11 a and the functional layer 11 b can be improved.

Referring to FIGS. 4 and 5, the functional layer may be disposed on the inner face of the door 14. In detail, the functional layer may be disposed on the inner face of the door 14 facing the cooking space 12 with the cooking space 12 being closed. The functional layer can improve the heat resistance, chemical resistance and stain resistance of the inner face of the door 14.

The door 14 may include a metal layer 14 a, and a functional layer 14 b on the metal layer 14 a.

The metal layer 14 a may be the base of the cavity portion.

Referring to FIG. 4, the functional layer 14 b may be disposed in direct contact with the metal layer 14 a.

Alternatively, referring to FIG. 5, the functional layer 14 b may be disposed in indirect contact with the metal layer 14 a. In detail, a buffer layer 14 c may be disposed between the metal layer 14 a and the functional layer 14 b. The buffer layer 14 c may comprise an adhesive layer. That is, the adhesion between the metal layer 14 a and the functional layer 14 b can be improved by the buffer layer 14 c.

The functional layer may be formed by coating the glass composition on the inner face of the cavity portion 11 or on the inner face of the door 14. In detail, the functional layer is coated on the inner face of the cavity portion 12 and on the inner face of the door 14 so that the inner face of the cavity portion 12 and the inner face of the door 14 have enhanced heat resistance, chemical resistance and stain resistance.

Hereinafter, the glass composition coated on the inner faces of the cavity portion and door of the cooking appliance will be described. The glass composition may be an enamel composition.

The glass composition according to the embodiment may include glass frit including P₂O₅, SiO₂, B₂O₃, Al₂O₃, ZrO₂, and group I-based oxide.

The P₂O₅ may be contained at up to about 30 wt % based on the total weight of the glass frit. Specifically, the P₂O₅ may be present in an amount of about 10 wt % to about 30 wt % based on the total weight of the glass frit. More specifically, the P₂O₅ may be present in an amount from about 15 wt % to about 25 wt % based on the total weight of the glass frit.

The P₂O₅ may be included in the glass composition to improve cleanability of the. Further, the P₂O₅ may be included in the glass composition to improve the hydrophilicity of the glass composition. Accordingly, as shown in FIG. 6, due to the presence of P₂O₅, the coating layer C containing the glass composition is hydrophilic. Thus, after water-soaking, water W can effectively penetrate the interface between the coating layer C and the contaminants P on the coating layer C, and the contaminants can be easily removed from the coating layer C.

If the P₂O₅ is included in a content smaller than about 10% by total weight of the glass composition, cleaning performance of the glass composition may be degraded. In detail, when the P₂O₅ is included in a content smaller than about 10% by total weight of the glass composition, the removal characteristics of contaminants such as chicken oil or saccharides may be deteriorated. If the P₂O₅ is included in a content larger than about 30% by total weight of the glass composition, the thermal properties of the glass composition may be degraded, the vitrification of the glass composition may be weakened, and the adhesion of the glass composition may be deteriorated.

The group I-based oxide includes at least one metal oxide selected from a group consisting of Li₂O, Na₂O and K₂O. Specifically, the group I-based oxide may include Na₂O, K₂O and Li₂O. That, the glass frit may contain all of Na₂O, K₂O and Li₂O.

The group I-based oxide may be contained up to about 30 wt % based on the total weight of the glass frit. Specifically, the group I-based oxide may be contained at from about 15 wt % to about 30 wt % based on the total weight of the glass frit. More specifically, the group I-based oxide may be contained at from about 20 wt % to about 25 wt % based on the total weight of the glass frit.

The group I-based oxide may be included in the glass composition to improve the cleanability and durability of the glass composition and reduce the melting temperature.

That is, the group I-based oxide can improve the cleanability of the glass composition together with the P2O5. For example, Na2O and K2O may be incorporated in the glass composition to improve the cleanability of the glass composition. Further, Li2O may be incorporated in the glass composition to improve the durability of the glass composition.

If the group I-based oxide is contained at less than about 15 wt % based on the total weight of the glass frit, the cleanability and durability of the glass composition may be degraded. Further, when the group I-based oxide is contained in an amount exceeding about 30 wt % based on the total weight of the glass frit, the thermal properties of the glass composition may be deteriorated and the vitrification may not be achieved.

P₂O₅, Na₂, K₂O, and Li₂O may form an alkali phosphate glass structure. Further, P₂O₅, Na₂, K₂O, and Li₂O may impart improved cleanability and durability to glass compositions according to the embodiments.

That is, since the glass frit contains P₂O₅, Na₂, K₂O, and Li₂O, the functional layer can be easily cleaned by water when the functional layer formed by the glass composition according to the embodiment is contaminated by food or the like.

SiO₂ may be present at up to about 40 wt % based on the total weight of the glass frit. Specifically, SiO₂ may be present in an amount of from about 20 wt % to about 40 wt % based on the total weight of the glass frit. More specifically, SiO₂ may be present in an amount from about 25 wt % to about 35 wt % based on the total weight of the glass frit.

SiO₂ may be incorporated in the glass composition to form a glass structure of the glass composition, to improve the skeleton of the glass structure, and to improve the acid resistance of the glass frit. In particular, due to SiO₂, the glass composition may have improved acid resistance. Further, SiO₂ is contained in the glass composition to improve the water resistance. In other words, SiO₂ is contained in the glass composition, to prevent the functional layer formed of the glass composition from absorbing water.

If SiO₂ content is less than about 20 wt % based on the total weight of the glass frit, the glass structure of the glass composition may be degraded, resulting in a reduction in durability, acid resistance, and water resistance of the functional layer. Further, if SiO₂ is contained in an amount exceeding about 40 wt % based on the total weight of the glass frit, the cleanability of the glass frit may be lowered.

B₂O₃ may be present at up to about 18 wt % based on the total weight of the glass frit. Specifically, B₂O₃ may be present in an amount of about 7 wt % to about 16 wt % based on the total weight of the glass frit. More specifically, B₂O₃ may be contained at from about 10 wt % to about 13 wt %, based on the total weight of the glass frit.

B₂O₃ can enlarge the vitrification region of the glass frit and appropriately control the thermal expansion coefficient of the glass composition according to the embodiment. Further, B2O3 is contained in the glass composition, allowing each component contained in the glass composition to be dissolved uniformly prior to firing.

When B₂O₃ is contained at less than about 7 wt % based on the total weight of the glass frit, the vitrification area is reduced and thus the glass structure is degraded, thereby reducing the durability of the functional layer. Further, if B₂O₃ content is greater than about 18 wt % based on the total weight of the glass frit, the cleanability of the glass frit may be degraded.

Al₂O₃ may be present at up to about 30 wt % based on the total weight of the glass frit. Specifically, Al₂O₃ may be present in an amount of about 15 wt % to about 30 wt % based on the total weight of the glass frit. More specifically, Al₂O₃ may be present in an amount from about 20 wt % to about 25 wt % based on the total weight of the glass frit.

The glass frit according to the embodiment can improve the cleaning performance of the sugar ingredient using the Al₂O₃.

In detail, when the glass composition including the glass frit according to an embodiment is coated as a functional layer on a cavity and/or door of a cooking appliance, and when the sugar component comes into contact with the functional layer, the caramelization reaction may occur due to sugar components.

That is, an oxidation reaction may occur between the sugar component and the functional layer. Accordingly, the characteristics of the functional layer may be deformed or deteriorated.

The Al₂O₃ acts to activate the acid catalytic reaction on the surface of the functional layer, and accordingly, may have an effect of inhibiting caramelization between the sugar component and the functional layer. Accordingly, the reliability of the functional layer can be improved by maintaining the characteristics of the functional layer.

If the Al₂O₃ is contained at less than about 15 wt % based on the total weight of the glass frit, the chemical resistance and durability of the glass composition may be reduced. Further, the characteristic of the functional layer may be deteriorated due to the caramelization reaction thereof with sugar components. Further, when the Al₂O₃ is contained in an amount of greater than about 30% by weight based on the total glass frit weight, the cleaning performance of the glass frit may deteriorate, and the process efficiency may be reduced.

ZrO₂ may be present at up to about 8 wt % based on the total weight of the glass frit. Specifically, ZrO₂ may be present at about 1 wt % to about 8 wt %, based on the total weight of the glass frit. More specifically, ZrO₂ may be present in an amount from about 3 wt % to about 5 wt % based on the total weight of the glass frit.

ZrO₂ may be incorporated into the glass composition to improve the chemical durability of the.

If ZrO₂ is contained at less than about 1 wt % based on the total weight of the glass frit, the chemical resistance and durability of the glass composition can be reduced. Further, when ZrO₂ is contained in an amount of more than about 8 wt % based on the total weight of the glass frit, the cleanability of the glass frit may be lowered, the vitrification may not be achieved, and the adhesion may be degraded due to increase in the melting temperature thereof.

That is, ZrO₂ may improve the chemical durability of the glass frit. In particular, Al₂O₃ and ZrO₂ may improve the chemical durability of the alkali phosphate glass structure formed by P₂O₅, Na₂O, K₂O and LiO₂ via structural stabilization.

The glass frit may contain at least one of TiO₂ and SnO. For example, the glass frit may contain both TiO₂ and SnO.

The TiO₂ and the SnO may be included in an amount of about 5% by weight or smaller with respect to the total glass frit weight. In detail, the TiO₂ and the SnO may be included in an amount of about 0.5 wt % to about 5 wt % with respect to the total glass frit weight. More specifically, the TiO₂ and the SnO may be included in an amount of about 1 wt % to about 4 wt % with respect to the total glass frit weight.

The SnO may function to appropriately adjust the surface tension of the coating film made of the glass composition, and improve the chemical resistance. Further, the TiO₂ can improve the hiding power of the glass composition according to the embodiment and improve the chemical resistance of the glass composition. That is, the TiO₂ can improve the hiding power of the coating layer of the glass composition coated as the functional layer.

If the TiO₂ and the SnO are included in a smaller content than about 0.5% by weight based on the total glass frit weight, the glass composition is reduced in the hiding power, such that the color of the base material or buffer layer may be visually recognized from outside when the glass composition is coated on the base material or buffer layer. Further, when the TiO₂ and the SnO are included in an amount of greater than about 5% by weight based on the total glass frit weight, the cleaning performance of the glass frit may be deteriorated.

The glass frit may further comprise fluorinated compounds. The fluorinated compound may be selected from NaF, CaF₂ or AlF₃. In detail, the fluorinated compounds can include NaF, CaF₂ and AlF₃. That is, the glass frit may contain all of NaF, CaF₂ and AlF3.

The fluorinated compound may function to properly control the surface tension of the coating formed by the glass composition. Further, the fluorinated compound may allow the vitrification area of the glass frit to be enlarged, and allow the melting temperature of the glass composition to be lowered.

The fluorinated compound may be contained at up to about 5 wt % based on the total weight of the glass frit. Specifically, the fluorinated compound may be contained at from about 0.1 wt % to about 5 wt %, based on the total weight of the glass frit. More specifically, the fluorinated compound may be present in an amount of about 1 wt % to about 4 wt % based on the total weight of the glass frit.

When the fluorinated compound is contained at less than about 0.1 wt % based on the total weight of the glass frit, the vitrification area is reduced and thus the glass structure is degraded, thereby decreasing the durability of the functional layer. Further, if the fluorinated compound is contained in an amount greater than about 5 wt % based on the total weight of the glass frit, the cleanability of the glass frit may be degraded.

The glass frit may further include at least one of Co₃O₄, NiO, Fe₂O₃ and MnO₂. In detail, the glass frit may include all of Co₃O₄, NiO, Fe₂O₃ and MnO₂.

The Co₃O₄, NiO, Fe₂O₃ and MnO₂ can improve the adhesion of the glass composition coated on the base. In other words, the Co₃O₄, NiO, Fe₂O₃ and MnO₂ can be an adhesion strengthening component that improves the adhesion when the glass composition containing the Co₃O₄, NiO, Fe₂O₃ and MnO₂ is coated on the base.

When the glass composition containing the Co₃O₄, NiO, Fe₂O₃ and MnO₂ is placed on the base, the glass composition can be coated directly on the base without an additional buffer layer therebetween.

For example, when forming the coating layer on a low carbon steel base, the adhesion enhancing component may react with the iron oxide of the base to improve adhesion between the coating layer and the base.

Thus, the glass composition can reduce the overall thickness of the coated cavity and/or door, and improve the process efficiency.

The Co₃O₄, NiO, Fe₂O₃ and MnO₂ may be included in an amount of about 5% by weight or smaller based on the total glass frit weight. In detail, the Co₃O₄, NiO, Fe₂O₃ and MnO₂ may be included in an amount of about 1% to about 5% by weight based on the total glass frit weight. More specifically, the Co₃O₄, NiO, Fe₂O₃ and MnO₂ may be included in an amount of about 2% to about 4% by weight based on the total glass frit weight.

Hereinafter, the present disclosure will be described in more detail based on glass composition manufacturing methods according to present embodiments and comparison examples. Those embodiments are merely an example to illustrate the present disclosure in more detail. Therefore, the present disclosure is not limited to those embodiments.

Present Example 1

The glass frit material was provided as in Table 1 below.

At this time, NH₄H₂PO₄ was used as the starting material for P₂O₅; Na₂CO₃, K₂CO₃, Li₂CO₃ were used as starting materials for Na₂O, K₂O, Li₂O respectively; CaCO₃, CaCO were used as starting materials for BaO and CaO, respectively; remaining components were the same as those shown in Table 1.

Then, after mixing the glass frit materials, the mixture was melted at a temperature of about 1400° C. for about 1 hour to about 2 hours and then quenched in a quenching roller to obtain glass cullet.

Subsequently, about 0.1 wt % to about 1 wt % of organic polysiloxane was added to the glass cullet, followed by milling and grinding in a ball mill for about 4 hours to about 6 hours. The crushed cullet passes through a 325 mesh sieve. As a result, the glass frits were formed to have a particle diameter of about 45 μm or less.

Then, the glass frits were sprayed onto a low carbon steel sheet having a size of 200×200 mm and a thickness of 1 mm or less using a corona discharge gun. In this connection, the voltage of the corona discharge gun was controlled to 40 kV to 100 kV. The amount of glass frits sprayed on the low carbon steel sheet was 300 g/m2.

Then, the low carbon steel having the glass frits sprayed thereon was fired at a temperature of 830° C. to 870° C. for 300 seconds to 450 seconds to form a functional layer on a face of the low carbon steel.

Present Example 2

The functional layer was formed in the same manner as the Example 1 except glass frit materials were provided as shown in Table 1 below.

Present Example 3

The functional layer was formed in the same manner as the Example 1 except glass frit materials were provided as shown in Table 1 below.

Present Example 4

The functional layer was formed in the same manner as the Example 1 except glass frit materials were provided as shown in Table 1 below.

Present Example 5

The functional layer was formed in the same manner as the Example 1 except glass frit materials were provided as shown in Table 1 below.

Comparison Example 1

The functional layer was formed in the same manner as the Example 1 except glass frit materials were provided as shown in Table 2 below.

Comparison Example 2

The functional layer was formed in the same manner as the Example 1 except glass frit materials were provided as shown in Table 2 below.

Comparison Example 3

The functional layer was formed in the same manner as the Example 1 except glass frit materials were provided as shown in Table 2 below.

TABLE 1 Composition (wt %) Example 1 Example 2 Example 3 Example 4 Example 5 SiO₂ 20.8 24.4 29.6 24.1 24.4 Al₂O₃ 17.9 17.7 18.2 18.1 17.7 P₂O₅ 24.5 24.4 24.1 23.2 24.4 K₂O 13.9 11.3 11.1 10.7 11.3 TiO₂ 0.9 — — — — NaF 1.9 1.8 1.9 1.9 1.8 Li₂O 0.5 0.9 1.0 0.9 0.9 B₂O₃ 9.4 7.2 4.9 11.8 7.2 Na₂O 5.7 6.4 6.4 6.3 6.4 Co₃O₄ 1.9 2.1 — — 2.1 NiO 0.4 0.4 — — 0.4 MnO₂ 0.4 0.6 — — 0.6 Fe₂O₃ 0.2 — — — — ZrO₂ 1.9 2.8 2.8 2.8 2.8

TABLE 2 Composition Comparison Comparison Comparison (wt %) Example 1 Example 2 Example 3 SiO₂ 32.6 29.5 25.6 Al₂O₃ 10.4 9.3 9.4 P₂O₅ 20.3 23.4 25.4 K₂O 7.1 9.9 12.2 NaF 2.3 2.0 2.0 Li₂O 1.1 1.0 1.0 B₂O₃ 14.6 13.0 13.2 Na₂O 8.0 7.2 7.0 ZnO — 1.7 — ZrO₂ 3.4 3.5 3.1

The applicants then proceeded to characterize the functional layers fabricated by the embodiments and comparison examples.

The softening points Td and the thermal expansion coefficients of the functional layers were measured, and the cleanability of each functional layer was measured by the cleanability test.

In order to measure the thermal properties of the glass, the pellet specimen was fired under the same conditions as those of the glass composition. After polishing both surfaces of the specimen in parallel with each other, the Td (softening point) and CTE (thermal expansion coefficient) were measured using a TMA (Thermo Machnical Analyzer) at a temperature increase rate of 10° C./min.

Further, the cleanability was measured by applying a 1 g of chicken oil as a contaminant on the surface of the enamel-coated test body (200×200 mm) using a brush. Thereafter, the test body to which the contaminant was applied was placed in a thermostat, and the contaminants were fixed to the body at a temperature of about 250 degree C. and for 1 hour.

After the fixation, the test body was caused to cool naturally, and then the degree of curing thereof was confirmed. Then, the test body was immersed in a water bath at 70° C. for 10 seconds to 10 minutes. Thereafter, the cured chicken oil and a cheery pie filling as sugar ingredients were wiped out with a wet cloth at a force of 3 kgf or less. A 5 cm-diameter bottom-flatted rod was used to homogenize the wiped area in the contaminated enamel surface. At this time, the number of wiping reciprocations as shown in Table 3 was measured and defined as the cleaning frequency. The evaluation indexes thereof were as shown in Table 4 and Table 5 below.

TABLE 3 Wiping frequency Performance Level equal to or smaller than 5 5 equal to or smaller than 15 4 equal to or smaller than 25 3 equal to or smaller than 50 2 larger than 50 1

TABLE 4 Softening Thermal expansion point (° C.) coefficient (×10⁻⁷/° C.) Example 1 509.5 115.8 Example 2 500.2 124.6 Example 3 491.7 137.2 Example 4 510.7 127.5 Example 5 494.9 112.0 Comparison Example 1 527.8 113.6 Comparison Example 2 563.2 117.1 Comparison Example 3 542.1 130.5

TABLE 5 Cleanability Example 1 5 Example 2 5 Example 3 5 Example 4 5 Example 5 5 Comparison Example 1 3 Comparison Example 2 3 Comparison Example 3 1

Referring to Table 4, it may be seen that the functional layers produced by the glass frits according to the Example 1 to Example 5 have a high softening point and thermal expansion coefficient. That is, the functional layers produced by the glass frits of the Example 1 to Example 5 has a softening point of about 500° C. or higher and a thermal expansion coefficient of about 100 (10⁻⁷/° C.) or more.

That is, it may be seen that the functional layers formed by the glass frits according to the Example 1 to Example 5 have improved durability and chemical resistance.

Further, referring to Table 5, it may be seen that the functional layers formed by the glass frits according to the Example 1 to Example 5 have improved cleanability compared to the functional layers formed by the glass frits according to the comparison examples 1 and 2.

That is, it may be seen that the functional layer made of the glass frit according to the embodiments has improved cleaning property while the softening point and the thermal expansion coefficient thereof remain at a similar level to those of the functional layer made of the free glass according to the Comparative Examples.

The glass composition according to an embodiment may have improved cleaning properties.

Thus, the cleaning property of the cooking appliance can be improved when using the functional layer made of the glass frit according to the embodiments. In detail, the cooking appliance can be easily cleaned only with water-soaking.

That is, although the inner face of the door and the inner face of the cavity portion are contaminated by food and organic substances generated during the cooking process while the cooking space defined in the cavity portion of the cooking appliance is closed, the inner face of the door and the inner face of the cavity portion may be easily cleaned only with the water-soaking.

Further, since the functional layer coated using the glass composition according to the embodiment is excellent in cleaning property, the interior of the cooking appliance can be easily cleaned with less energy.

Further, since the functional layer can be directly coated on the back surface of the cavity and door without a separate buffer layer therebetween, the process efficiency can be improved while minimizing the increase in the thickness of the cavity and door due to the coating of the functional layer.

Further, since the glass composition according to the embodiment has a softening point and a thermal expansion coefficient higher than a certain temperature, the glass composition can withstand cooking and washing at a high temperature for a long time.

Further, when in the glass composition according to the embodiment, the weight percentage of Al₂O₃ is appropriately controlled, the caramelization reaction that occurs when the specific component (e.g., sugar component) contacts the functional layer made of the glass composition can be reduced.

Therefore, it is possible to prevent the characteristics of the functional layer from being changed or deteriorated due to oxidation reaction such as caramelization reaction.

While the embodiments have been described above, the present disclosure is not intended to limit the embodiments. Those skilled in the art will recognize that various changes and modifications may be possible without departing from the essential characteristics of the present invention. For example, each component specifically illustrated in the embodiment may be modified. It is also to be understood that the various modifications and variations are intended to be included within the scope of the present disclosure as defined in the appended claims. 

What is claimed is:
 1. A glass composition comprising a glass frit containing P₂O₅, SiO₂, B₂O₃, Al₂O₃, ZrO₂ and a group I-based oxide, wherein the P₂O₅ is contained in an amount of 10 to 30% by weight based on a total weight of the glass frit, wherein the SiO₂ is contained in an amount of 20 to 40% by weight based on the total weight of the glass frit, wherein the B₂O₃ is contained in an amount of 5 wt % to 18 wt % based on the total weight of the glass frit, wherein the Al₂O₃ is contained in an amount of 15 to 30% by weight based on the total weight of the glass frit, wherein the ZrO₂ is contained in an amount of 1 wt % to 8 wt % based on the total weight of the glass frit, wherein the Group I-based oxide is contained in an amount of 15 to 30% by weight based on the total weight of the glass frit.
 2. The glass composition of claim 1, wherein the glass frit further contains at least one of TiO₂ and SnO.
 3. The glass composition of claim 2, wherein the glass frit contains both TiO₂ and SnO, wherein each of TiO₂ and the SnO is contained in an amount of 0.5 to 5% by weight based on the total weight of the glass frit.
 4. The glass composition of claim 1, wherein the glass frit further contains a fluorine (F) compound, wherein the fluorine (F) compound is contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the glass frit.
 5. The glass composition of claim 4, wherein the fluorine (F) compound includes at least one metal oxide selected from NaF, CaF₂ and AlF₃.
 6. The glass composition of claim 1, wherein the glass frit further contains an adhesion enhancing component, wherein the adhesion enhancing component is contained in an amount of 1 wt % to 5 wt % based on the total weight of the glass frit.
 7. The glass composition of claim 6, wherein the adhesion enhancing component includes at least one metal oxide selected from Co₃O₄, NiO, Fe₂O₃ and MnO₂.
 8. A cooking appliance comprising: a cavity having a cooking chamber defined therein; a door for selectively opening and closing the cooking chamber; and at least one heating source for supplying heat for heating food in the cooking chamber, wherein at least one of the cavity and door includes a metal base and a functional layer disposed on the metal base, wherein the functional layer comprises a glass frit containing P₂O₅, SiO₂, B₂O₃, Al₂O₃, ZrO₂ and a group I-based oxide, wherein the P₂O₅ is contained in an amount of 10 to 30% by weight based on a total weight of the glass frit, wherein the SiO₂ is contained in an amount of 20 to 40% by weight based on the total weight of the glass frit, wherein the B₂O₃ is contained in an amount of 5 wt % to 18 wt % based on the total weight of the glass frit, wherein the Al₂O₃ is contained in an amount of 15 to 30% by weight based on the total weight of the glass frit, wherein the ZrO₂ is contained in an amount of 1 wt % to 8 wt % based on the total weight of the glass frit, wherein the Group I-based oxide is contained in an amount of 15 to 30% by weight based on the total weight of the glass frit.
 9. The cooking appliance of claim 8, wherein the glass frit further contains at least one of TiO₂ and SnO, and further contains a fluorine (F) compound, wherein each of TiO₂ and the SnO is contained in an amount of 0.5 to 5% by weight based on the total weight of the glass frit, wherein the fluorine (F) compound is contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the glass frit.
 10. The cooking appliance of claim 8, wherein the glass frit further contains an adhesion enhancing component, wherein the adhesion enhancing component is contained in an amount of 1 wt % to 5 wt % based on the total weight of the glass frit, wherein the adhesion enhancing component includes at least one metal oxide selected from Co₃O₄, NiO, Fe₂O₃ and MnO₂. 